The invention relates to reducing gear-train of watches having an analog display driven by a step-by-step motor whose control frequency is a fraction of a minute, for example a second.
As known, such watches are generally equipped with a reducing gear-train having a driving pinion meshing with a second wheel having a second pinion meshing with a third wheel which in turns has a pinion meshing with a centrally-disposed great wheel or "centre-wheel." The latter wheel, which serves for the display of minutes, makes one revolution per hour, it being understood that the wheels and pinions of the same name are solidly fixed and coaxial.
As known, display of the hour is obtained by means of a plane differential gear train having, coaxially with the great or "centre" wheel, a cannon-pinion meshing with a dial-train wheel having a pinion which meshes with an hours wheel, which is also coaxial with the centre wheel.
To permit setting of the time, when the dial train is directly driven by the user (by means of a time-setting stem) and to avoid the entire great-train from being driven, a friction clutch is provided between the cannon pinion and the centre-wheel arbor. This is generally achieved by indenting the cannon pinion on the centre-arbor.
The most bulky element in any electronic watch with an anlog display is generally the energy source, i.e. the cell. To obtain the movement with as small a diameter as possible, for example a movement for ladies watches, the manufacturer is led to place the cell as close as possible to the centre. Because of this, with a gear-train such as that previously described, two wheels must pass under or over the cell, which necessarily brings the height of the movement to a value incompatible with an elegant exterior.
So as to alleviate this disadvantage, there have been proposals for cells of non-circular shape and dimensioned to enable two central wheels to pass beside the cell; this arrangement has enabled movements of small dimensions which can be fitted in shaped ladies cases to be obtained. To the contrary, for round exteriors, the benefit of this construction is lost and round exteriors are generally too large for ladies watches.
Another disadvantage of the previously described reduction train is the fact that by three gear trains one must obtain a great reduction which, in surface area, is also incompatible with a ladies watch movement of small dimensions having pinions which must remain within dimensions (module and diameter, compatible with mass production.
By way of example, with a motor supplied at 1 Hz and having 8 steps per rotation, the reduction that must be obtained for the minutes arbor (centre wheel) is 1/450.